This depends by what you mean "resilient fighters". For example, Switzerland has one of the best combat record against defeating larger nations and armies (in fact, almost every nation they fought is bigger than they are). However, in terms of the traditional meaning of "resilience", the Swiss person on average aren't impressive. Himalayan Sherpas live on much higher altitude than they do, meaning the Swiss aren't necessarily the best mountain survivalists in the world. In addition, the presence of Swiss people in fighting sports such as the UFC is low compared to lower countries like the Netherlands or Britain for example.

If you meant to ask whether nations on difficult condition climates produced better armies on average, this is not true. For example, it is popularly believed that nations on steppe regions produce better cavalrymen than settled city nations. However, by the turn of the 1800s for example, Russian pioneers whom come from settled cities themselves were able to traverse Siberia and engage the natives there with relatively moderate difficulty. By that same century, European light cavalry regiments in the form of Hussars have proven to be much more popular and effective than Cossack cavalry regiments , who themselves originate from steppes.

You also have counterexamples to Afghanistan that prove otherwise that mountain settlers are any better at fighting than lowland settlers. Take the Gurkhas of the regions of Nepal for example, who despite having a good reputation as soldiers, had better numerical advantages, protection of the large Himalayan mountain ranges and access to resources in their home territory, lost to the British in the Anglo-Nepal war of 1814.

It seem that training, equipment, weaponry and the general condition of the soldiers themselves have a greater effect on the true effectiveness of an army rather than their place of origin. In the case of Afghanistan, failing morale and lack of support overseas seem to be the primary reason why they were able to repel a couple of larger nations in the 19th and 20th centuries.